1. Field of the Invention
The present invention relates to a method and an apparatus or a syringe for collecting a blood sample from a human or animal blood vessel, such as an artery or vein.
2. Description of the Prior Art
Arteries are blood vessels through which blood is passed from the lungs to the tissues in human or animal bodies, and the blood is passed from the tissues back to the lungs through the veins. Therefore, artery blood contains oxygen being used for combustion or oxidation processes in the tissues, and the blood in the veins transports carbon dioxide generated in the oxidation processes back to the lungs. The object of certain blood examinations -- the so-called acid-base tests -- is i.e., to establish a value indicating the lung effectively or the oxygen contents of the artery blood.
Several methods for collecting blood samples to be used for the said tests are known. Thus, it is known to collect a blood sample from an artery by means of a syringe of the type having a cylinder provided with a hollow needle or hypodermic needle and a piston displaceably mounted in the cylinder. According to the said known method the main artery in the upper arm (arteria brachialis), in the wrist (arteria radialis), or in the thigh (arteria femoralis) is punctured by means of the syringe needle while the piston is in its advanced position, and thereafter a suitable blood sample is sucked into the syringe cylinder by retracting the piston. However, the arteries are rather deep-lying and not easily accessible, and therefore it may be rather difficult to hit the artery with the syringe needle and it cannot be ascertained whether the artery has been properly punctured by the needle before the piston of the syringe is being retracted in relation to the cylinder. In case no proper puncturing has taken place a new attept must be made to hit the artery. Furthermore, it is difficult to keep the syringe needle completely still after its puncture of the artery, partly because the position of the hands holding the syringe have to be changed, and partly due to the friction between the piston and the syringe cylinder. Therefore, during retraction of the piston the needle point may be moved to such an extent that it slips out of the artery and/or increases the size of the hole made in the artery wall. Due to these facts collection of blood samples from arteries by the known method described is painful as well as subject to risk, and therefore in a number of countries such collection of blood samples is allowed to be made by doctors only.
In order to avoid the painful and risky artery puncture to the farthest possible extent it is sometimes replaced by the so-called capillary tube technique involving local increase of blood circulation in a tissue (for example in a heel, an ear, or a finger tip) by massage, the action of heat, or in another manner, whereby the oxygen content of the blood in the said tissue is increased so that that the blood becomes similar to artery blood. In accordance with said capillary tube technique a blood sample is collected by making a small incision in the said tissue in order to create a small drop of blood which is sucked into a capillary tube by inserting one end thereof into a drop of blood. By making the previously mentioned tests of blood samples collected by the capillary tube technique it is possible to obtain an acceptable accuracy compared with corresponding measurements made on real artery blood, but naturally by using the capillary tube technique it is possible to collect only relatively small blood samples which are insufficient to be treated in modern fully automatic or semi-automatic blood analyzing apparatuses. Furthermore, it is not possible to use the capillary tube technique in collecting blood samples from patients suffering from shock, because the blood vessels in the tissues are constricted in such a shock condition.
It is also known to avoid the problems in connection with artery puncture by making the measurements desired on vein blood instead of on artery blood and thereafter on the basis of empirically determined normal values try to compensate for the errors appearing in the measuring results due to the use of the vein blood instead of artery blood. The lastmentioned method has the advantage that the blood sample collecting technique for vein blood is relatively simple, without risk, and substantially without pain, but the results obtained by the measurements are of course rather inaccurate.